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Q. I have been getting spontaneous nosebleeds. What are some of the causes, and what can be done to prevent them?

A. In the movies, nosebleeds are usually the badge of a good fight or the herald of some mysterious and painful death.

When nosebleeds happen suddenly in real life, they can be alarming. But Raj Krishnamurthy, vice chair of outpatient medicine at Boston Medical Center, says that nosebleeds are rarely a sign of a serious problem. This time of year, the culprit is usually more pedestrian: dry air.

The mucous membranes that line the nasal passages need constant moisture; when they dry out, blood vessels underneath become irritated and more likely to bleed. “It’s like when your lips and inner mouth are dry and cracked,’’ she says. Nosebleeds are more common during winter months when the air is cold and dry, or in the heat of summer.

Taking nasal decongestants for colds can also dry out the nasal passages. More rarely, nasal sprays for allergies can lead to nosebleeds. In these situations, Krishnamurthy says, you can help prevent further nosebleeds by using a saline nasal spray to keep the nasal passage moist, or by applying petroleum jelly directly to the inside of the nose. Humidifiers can also help.

While the vast majority of nosebleeds are harmless, Krishnamurthy says that in rare instances they can be a sign of an underlying health problem such as high blood pressure, though no one knows exactly why hypertension causes the nose to bleed. They can also be a symptom of a more serious bleeding disorder, but in these cases nosebleeds are usually accompanied by other symptoms like bleeding gums and bruising.

Taking aspirin or other blood thinners regularly, while it doesn’t usually cause nosebleeds, can make them last longer when they do occur.